1. Field of the Invention
The present invention relates to a reflection type Faraday rotator which constitutes an optical isolator and an optical circulator used in optical-fiber communication, optical recording, optical measuring systems, and so forth.
2. Description of the Related Art
In optical fiber communication systems using semiconductor lasers as light sources, especially in optical systems by the high-speed digital transmission and the analog direct modulation method, when reflected light from optical connector junctions, optical circuit components, etc., used in optical-fiber circuits returns to the semiconductor lasers or optical amplifiers, frequency characteristics are degraded, and noises are generated, thereby giving difficulty to high quality transmission. In order to eliminate the reflected light, an optical isolator has been used.
The aforementioned optical isolator is composed mainly of a polarizer and an analyzer both of which transmit light of a specific plane of polarization only, a Faraday rotator for rotating the plane of polarization for the light by 45 degrees, and a permanent magnet for applying a magnetic field to the Faraday rotator.
The performance of the optical isolator depends primarily on the Faraday rotator among the above-mentioned constituents. The Faraday rotator is required to have a small length of an element for rotating the plane of polarization by 45 degrees and a large light transmittance. As the Faraday rotator, there are a bulk single crystal (thickness of about 2 mm) of yttrium iron garnet (YIG), a thick film single crystal (thickness of about several hundred micrometers) of bismuth-substituted rare-earth iron garnet (BiYIG), in which bismuth having a large magneto-optical performance index has substituted for part of yttrium, and so forth. Recently, the BiYIG tick film single crystal advantageous in downsizing the optical isolator has been used in many cases.
Moreover, recently, there has been proposed a Faraday rotator (magneto-optical member) made of one-dimensional magneto-photonic crystal which brings about the enhancement of a magneto-optical effect (the Faraday effect is a kind of the magneto-optical effect) due to the localization of light. Since the Faraday rotator (magneto-optical member) made of one-dimensional magneto-photonic crystal can provide a large Faraday rotation angle with a thickness of as small as several micrometers, optical isolators are expected to be significantly downsized and manufactured at a reduced cost.
The one-dimensional magneto-photonic crystal is structured such that at least one kind of dielectric thin film and a magneto-optical thin film are laminated so as to form a Fabry-Perot resonator structure. It has been recognized that the magneto-optical effect can be significantly increased in particular when a magneto-optical thin film is sandwiched between two periodic dielectric multilayer films each structured such that two kinds of dielectric thin films having refractive indexes different from each other are laminated.
However, in the above-described simple structure, when the number of layers of the periodic dielectric multilayer films is increased for increasing a Faraday rotation angle, light transmittance is lowered. It has been revealed that this problem can be solved by stacking a plurality of Fabry-Perot resonators so as to sandwich a dielectric thin film. However, this method increases significantly the number of layers and the manufacturing cost.
Under the circumstances, the inventors have invented a Faraday rotator (reflection type laminate film Faraday rotator) which has a metal reflection film formed on the top or bottom surface of the laminate film in the direction of lamination, and in which light is made incident on a side having the metal reflection film not formed on and is reflected at an opposite side where the metal reflection film is present. Then, the optimization of the structure of the reflection type Faraday rotator was examined. As a result, satisfactory magneto-optical characteristics could be obtained with a significantly smaller number of layers compared with a conventional transmission type Faraday rotator. The reflection type Faraday rotator does not have to include a metal reflection film for achieving reflection function. Satisfactory reflection characteristics can be given by appropriately structuring the dielectric multilayer films.
FIG. 14 shows an example of the configuration of an optical isolator using a reflection type laminate film Faraday rotator. FIG. 15 shows the film structure of the reflection type laminate film Faraday rotator.
In FIG. 14, an optical isolator 1 comprises a polarizer 2 and an analyzer 3 both of which transmit only light component having a specific plane of polarization, a reflection type laminate mm Faraday rotator 4 which rotates the plane of polarization of light by 45 degrees, and a permanent magnet (not shown) for applying a magnetic field to the Faraday rotator 4. In FIG. 15, the Faraday rotator 4 comprises a substrate 5, and a laminate film 6 formed on the substrate 5. The laminate film 6 comprises two periodic dielectric multilayer films 9 and 10 each comprising two kinds of dielectric thin films 7 and 8, a magneto-optical thin film 11 sandwiched between the two periodic dielectric multilayer films 9 and 10, and a metal reflection film 12 formed on a side of the periodic dielectric multilayer film 9 near the substrate 5.
From the standpoint of increasing a communication capacitance, the wavelength division multiplexing system (WDM), in which light with a plurality of wavelengths is transmitted through one optical fiber, is may used. The wavelength interval ranges from 0.4 to 1.6 nm. Accordingly, a wavelength band of 60 nm is required for WDM of 40 channels. Thus, an optical isolator and, also, a Faraday rotator are required to have a wavelength band of at least 60 nm. The bandwidth is further increasing and the WDM system is under development.
However, since the Faraday rotator including a laminate film has a small wavelength range for operation, it is difficult for one Faraday rotator to cover a wide wavelength range though it can act on a specific wavelength. For increasing the wavelength bandwidth of the laminate film Faraday rotator, the inventors have proposed, in Japanese Patent Application No. 2000-312003, an optical device which has a function of changing the incidence position and angle of light incident on a laminate film, depending on the wavelength of the incident light. Specifically, the optical device comprises a prism spectroscope or diffraction grating, which requires special design and processing technique for production, pushing up cost of the Faraday rotator.
And, further reduction of the number of layers of the laminate film which constitutes the Faraday rotator is required.
The present invention was made in light of the above, and it is therefore an object of the present invention to provide a reflection type Faraday rotator which can increase the wavelength bandwidth and reduce the cost.
According to a first aspect of the present invention, a reflection type Faraday rotator comprises a plurality of sub-rotators which each include a laminate Mm at least comprising an magneto-optical thin film and at least one kind of dielectric thin film, have reflection characteristics, have wavelength characteristics different from one another, and which are arranged so that reflected light from one sub-rotator is made incident on another sub-rotator in sequence.
According to a second aspect of the present invention, a reflection type Faraday rotator comprises at least three sub-rotators which each include a laminate film at least comprising a magneto-optical thin film and at least one kind of dielectric thin film, have reflection characteristics, wherein at least two sub-rotators have an identical wavelength characteristic but a different wavelength characteristic from at least one sub-rotator, and which are arranged so that reflected light from one sub-rotator is made incident on another sub-rotator in sequence.
According to a third aspect of the present invention, a reflection type Faraday rotator comprises at least one sub-rotator which each include a laminate film at least comprising a magneto-optical thin film and at least one kind of dielectric thin film and having a constant thickness, and which has a reflection characteristics, and at least one reflector which is disposed so as to face the laminate film thereby being adapted to receive and reflect reflected light from the sub-rotator.
According to a forth aspect of the present invention, a reflection type Faraday rotator comprises at least one sub-rotator which each include a laminate film at least comprising a magneto-optical thin film and at least one kind of dielectric thin film and having a thickness changed linearly or stepwise, and which has reflection characteristics, and at least one reflector which is disposed so as to face the laminate film of the sub-rotator thereby receiving and reflecting reflected light from the sub-rotator.
According to a fifth aspect of the present invention, a reflection type Faraday rotator comprises at least one sub-rotator which each include a laminate film at least comprising a magneto-optical thin film and at least one kind of dielectric thin film and having a constant thickness and which has reflection characteristics, and at least one reflector which is disposed so as to face the laminate film of the sub-rotator at a predetermined inclination angle with respect to the laminate film thereby receiving and reflecting reflected light from the sub-rotator.
According to a sit aspect of the present invention, a reflection type Faraday rotator is provided in which a transparent substrate is sandwiched between the laminate film and the reflector.
According to a seventh aspect of the present invention, a reflection type Faraday rotator is provided in which a sub-rotator which includes a laminate film at least comprising a magneto-optical thin film and at least one kind of dielectric thin film, and which has reflection characteristics, is provided instead of the reflector.
According to an eighth aspect of the present invention, a reflection type Faraday rotator is provided in which the laminate film comprises two periodic dielectric multilayer films each comprising two kinds of dielectric thin films with different refractive indexes, a magneto-optical thin film sandwiched between the two periodic dielectric multilayer films, and a metal thin film formed on a side of one periodic dielectric multilayer film of the two opposite to a side in contact with the magneto-optical thin film.